The present invention relates to magnetic hard disk drives. More specifically, the present invention relates to a method of measuring the fly height of magnetic heads over magnetic data storage media by electrostatic force.
FIG. 1 illustrates a hard disk drive design typical in the art. Hard disk drives are common information storage devices essentially consisting of a series of rotatable disks 101, or other magnetic storage mediums, that are accessed by magnetic reading and writing elements. These data transferring elements, commonly known as transducers 102, are typically carried by and embedded in a slider body that is held in a close relative position over discrete data tracks formed on a disk 101 to permit a read or write operation to be carried out. In order to properly position the transducer 102 with respect to the disk surface, an air bearing surface (ABS) formed on the slider body experiences a fluid air flow that provides sufficient lift force to “fly” the slider and transducer above the disk data tracks. The high speed rotation of a magnetic disk generates a stream of air flow or wind along its surface in a direction substantially parallel to the tangential velocity of the disk. The air flow cooperates with the ABS of the slider body which enables the slider to fly above the spinning disk. In effect, the suspended slider is physically separated from the disk surface through this self-actuating air bearing.
FIG. 2 illustrates a slider flying at a fly height over a magnetic disk. Some of the major objectives in ABS designs are to fly the slider and its accompanying transducer 202 as close as possible to the surface of the rotating disk, and to uniformly maintain that constant close distance regardless of variable flying conditions. The height or separation gap between the air bearing slider and the spinning magnetic disk is commonly defined as the fly height. In general, the mounted transducer or read/write element flies only approximately less than one micro-inch (25.4 nm) above the surface of the rotating disk. The fly height of the slider is viewed as one of the most critical parameters affecting the magnetic disk reading and recording capabilities of a mounted read/write element. The fly height of the slider also directly impacts the mechanical integrity and magnetic performance of a disk drive. A relatively small fly height allows the transducer to achieve greater resolution between different data bit locations on the disk surface, thus improving data density and storage capacity. With the increasing popularity of lightweight and compact notebook type computers that utilize relatively small yet powerful disk drives, the need for a progressively lower fly height has continually grown.
Various methods exist for measuring the fly height of a slider. One such method is optical fly height measurement. Optical fly height measurement is based on the interference of light reflected off the slider ABS and disk surface. It requires a transparent disk, and poses a great danger of electrostatic discharge (ESD) damage to the delicate read-write transducers. As a result, optical fly height measurement is typically only performed on a small fraction of sliders, as part of statistical process control.
Fly height measurement by read-back signal is another method of measuring the fly height of a slider. Fly height measurement by read-back signal is based on the Wallace equation of spacing loss. By comparing the amplitudes of read-back signals under two distinct conditions, the spacing variations can be calculated. However, the slider fly height must be known under at least one of the two distinct conditions in order to know the actual fly height. Thus, fly height measurement by read-back signal usually requires a “touch-down” process, in which the fly height of the slider is brought down to, or near, zero. Measurement of the slider fly height upon touch-down provides a reference fly height for measuring the slider fly height at the other distinct condition, and permits the actual fly height to be measured. The touch-down process, however, causes wear in the slider-disk interface and is time-consuming. Moreover, the magnetic spacing between the read-write transducer and the disk at touch-down is uncertain due to the inclusion of debris and lubricant as well as due to slider vibration. The result does not correlate well to actual slider reliability.
Recently, fly height adjustment by thermal expansion has been adopted by the industry. An electrical heater embedded in the slider, in the proximity of the read-write transducer, causes the transducer to protrude out of the originally flat ABS, toward the disk. The nominal fly height may be raised somewhat before activation, then reduced as necessary by the heater to perform read-write activities. With this technology, slider fly height tolerance can be relaxed somewhat. However, fly height control remains critical in the drive production, to minimize the number of low-flyers (sliders that fly too low and which cannot be salvaged by the heater) and high-flyers (sliders that fly too high and which require excessive heat, thus wasting electricity and shortening the read sensor lifespan). The “touch-down” is accelerated and simplified with the heater, compared to sliders without the heater. Nevertheless, the fly height at touch-down remains uncertain. Further, wear and tear on the slider and the disk may prevent the touch-down process from being performed repeatedly. Accordingly, it is difficult to optimize the magnitude of heater protrusion in-situ in portable devices, in which the fly-height changes frequently with ambient pressure and temperature.
Fly height may also be adjusted by electrostatic force. A voltage is applied between the slider body and the disk. Compared with the thermal expansion method, the electrostatic force method requires less power. Also, adjusting the fly height by electrostatic force does not heat up the read-write sensor. However, there is a risk of arcing if the voltage is excessive. Regardless of the method of fly height adjustment, the fly height itself can be measured either by read-back signal or by the capacitance between the slider and the disk. In either case, an undesirable “touch-down” is necessary.
Thus, what is needed is an improved method of measuring the slider fly height, without a transparent disk, and without “touch-down.”